As for a conventional pulsation reducing apparatus, there have been utilized a diaphragm type, a bladder type and a bellows type apparatus or the like. However, such apparatuses are set to be kept at high initial set pressure values and operate ineffectively in a low or a negative pressure range. Further, the aforementioned apparatuses have high spring constants for dampening pulsation and are unable to dampen any minute pulsation. Furthermore, in terms of a material strength, none of them is usable in a low temperature range of, e.g., minus several tens degree. In addition, in case of a gas pressurization-typed pulsation reducing apparatus, it is difficult to uniformly maintain a liquid surface height in a tank due to a dissolution of sealed gas in liquid, a separation of gas from liquid, e.g., a vaporization of liquid or dissolved gas and a pressure change of circulating liquid or the like.
Japanese Utility Model Laid-open No. H6-403383 discloses therein a water level controller of an air chamber, for reducing a pulsation in a quite different manner from those of the aforementioned apparatuses. In such water level controller of the air chamber, the inconvenience for supplementing air is reduced by lengthening intervals for supplementing air in the air chamber with a reciprocating pump, to thereby reduce a labor associated with such operation and maintain a stable operation for a long period of time.
Further, in case of such controller, an air chamber such as an air inlet chamber where air is sealed or the like is employed as a main chamber. The main chamber is provided with a water level control pipe through which liquid of the chamber is overflowed to be discharged to thereby control the level of the height of the liquid, wherein the water level control pipe is connected to an auxiliary chamber where air is sealed. Initially, the liquid does not overflow from the water level control pipe and, also, a pulsation reducing function, a cushioning function and a volume compensating function are normally performed. However, when the air is dissolved into the liquid to increase the water level, the liquid overflows through the water level control pipe into the auxiliary chamber. Thus, an air as much as the liquid is pressed and supplemented substitutively into the main chamber, thereby ensuring a minimum air volume required for a long period of time.
Meanwhile, in case of the pulsation reducing apparatus disclosed in Utility Model Laid-open No. H6-403383, the main chamber is positioned higher than the auxiliary chamber in a positive pressure line of a discharging side of the pump. Accordingly, liquid increased by air dissolved in the main chamber overflows into the auxiliary chamber and, then, the liquid decreased by the overflow is substituted with air in the auxiliary chamber. As a result, a pressure increase in the main chamber, which is caused by the vaporization of liquid or the like, is not considered.
However, in case a target object is cooled on a mounting table and then maintained at a uniform temperature during an inspection using the inspection apparatus for the target object such as, e.g., a semiconductor wafer or the like, a coolant circulating in the mounting table is easily vaporized in the main chamber. Further, since a vaporization volume of liquid exceeds a dissolution volume of air into liquid, a gas pressure in the main chamber increases and, also, the pulsation reducing function deteriorates.